This invention relates to the manufacture of polishing pads having optically transmissive windows for monitoring the chemical mechanical planarization process performed on semiconductor wafers. In particular, the present invention is directed to a novel method for the formation of optical windows in a large volume molded assembly containing a multiplicity of polishing pads and the product formed thereby.
In semiconductor manufacturing, the device layers formed by deposition of materials on semiconductor wafers requires the use of planarization processes to control the thickness of deposited films and to restore planarity to the operating surface for succeeding lithographic operations. The entire process is referred to as chemical mechanical planarization (CMP) and one of the steps of the CMP process includes the use of polishing pads to polish and render uniform the different elevational features of the exposed surface. During the conduct of the polishing process it is desirable to determine when the current stage of the process should be halted.
Typically, the decision regarding changing or halting the processing step utilizes optical detection techniques to read the thicknesses of the transparent films formed on the semiconductor wafer or to read the reflection transitions for opaque films formed thereon. In either method of monitoring the processing of semi-conductor wafers, the measurements are typically made through transparent or translucent windows formed in the polishing pads.
The windows contained in the polishing pads are typically formed by placing transparent polymer plugs into foamy type opaque polymer materials. The result is the formation of a pad having one or more relatively small area regions which are transparent and surrounded by adjacent opaque regions. The adjacent regions are relied on to carry forward with the chemical/mechanical polishing activity in combination with an applied slurry.
One approach to forming a polishing pad having a transparent window therein is disclosed in U.S. Pat. No. 5,893,796 wherein a transparent plug is preformed as a solid insert that is then molded into the pad. The reference teaches the securing of the plug in a preformed hole by adhesive bonding to the polishing pad. This technique has been found to generate problems in that the window material is different than the surrounding pad material and the cracks at the window interface allow contamination to build up between the pad and the window. Since the pad is typically formed from a foamed resin, the formation of a hole in the polishing pad to receive a preformed window results in an irregular interface. Any dimensional mismatch at a point on the interface can result in leakage during use. There is also a difficulty of dressing or grinding the pad surface in that dissimilar materials will be removed at different rates causing crowning or dishing of the window. Pad protrusions from crowning will affect the uniformity of polishing. Dishing will affect the window optics by allowing for waste materials to accumulate and occlude the light signal.
Another approach to forming windows has involved casting transparent and translucent polymers into holes cut into polishing pads. A problem with this technique is that it is difficult and costly to form the window with surfaces flush to the surfaces of the polishing pad due to resin flow and shrinkage. Alternatively, U.S. Pat. No. 5,605,760 teaches the use of a transparent pad to facilitate the determination of the endpoint in processing.
In U.S. Pat. No. 6,171,181, the formation of a one-piece molded article for use as a polishing pad is disclosed. The polishing pad is formed by solidifying a flowable polymeric material and using selective cooling rates so that one region remains transparent after hardening while the surrounding regions are cooled at a slower rate to become relatively opaque. The reference continues on by pointing out that since the transparent region and the opaque region are integrally molded from the same polymeric material, the boundary is not a distinct structural transition. The utilization of differential cooling results in a pad having regions of a crystalline phase and a region having a combination of crystalline and amorphic phases to provide different light transmissive characteristics.
The manufacturing process described in the above-noted patent reference generates individual polishing pads which are relatively thin having a thickness dimension of the order of 0.05 to 0.08 inches. The process utilizes a mold designed with an isolated temperature zone having an independent temperature control. The zone establishes an approximate shape and location of the desired transparent window. Thus, the physical characteristics and location of the window so formed are not always predictable. Furthermore, the process requires an especially designed mold to manufacture individual polishing pads.
Accordingly, the present invention is directed to a method of making a multiplicity of polishing pads in a large volume molded cake which can be skived into individual pads after the formation of the transparent window. Furthermore, the present method enables the cross-sectional area or shape of the viewing window to be defined and predictable. The bulk fabrication process utilizes a structural sleeve interposed between the opaque and transparent polymer resins to define the transparent window. The sleeve is formed from a polymer that cross-links to the polishing pad material and to the window material during curing. As a result, the junctures of the sleeve with viewing window and adjacent pad are sufficiently strong to essentially eliminate cracks or boundary separations in which accumulation of debris would otherwise occur during normal polishing operations.
The present invention is directed to a method of forming polishing pads having transparent windows and the polishing pads so formed The polishing pads are intended for use in connection with CMP processing of semiconductor wafers. The present invention is well suited for the fabrication of a large volume blank containing at least one transparent window extending therethrough thus enabling a blank capable of providing a multiplicity of polishing pads with windows to be formed in a single molding operation. The large volume blank is then subdivided to form a multiplicity of relatively thin individual polishing pads.
The present method of making a polishing pad of the type containing a light-transmissive window therein includes the steps of providing a thin-walled sleeve of a first polymeric resin. A second polymeric resin contained in the sleeve forms the window having light-transmissive capability. The sleeve is sited in a large volume mold that is filled with a third polymeric resin, typically the foaming resin used to form the blank or cake. The first and third resins are partially cured to promote cross-linking between the sleeve and surrounding blank. The second resin is added to fill the sleeve. The mold is used to define the contour of the cake that will later be skived into a multiplicity of individual polishing pads. After filling the mold with the three resin components forming the resultant pad, a curing step takes place which concurrently cures the components and forms a unitary blank suitable for subdivision into a number of individual polishing pads. The entire assembly is fully cured with the actual time and temperature parameters being determined primarily by the particular resins employed. After curing, the molded blank is removed. During the partial and full curing steps, the resins at the different interfaces between the structural sleeve and the resins bounding either side thereof become cross-linked. This cross-linking of polymers essentially eliminates any gaps or structural discontinuities between the structural sleeve and its adjacent elements.
Although a preformed and partially cured sleeve may be utilized, the preferred manner of practicing the present method utilizes a sleeve that is supported in the mold with a mandrel. After the third resin is poured into the mold and partially cured, the mandrel is removed from the mold. Then, the transparent second resin is used to fill the sleeve. The entire assembly is then fully cured while residing in the mold.
The sleeve can be formed by the immersion of a mandrel having a length at least equal to the height of the molded cake in a liquid phase of the first resin. Alternatively, the mandrel also can be wrapped with a thick film of partially cured first resin to form the sleeve. In both cases, the sleeve is filled with the optically-transmissive second resin and subjected to a partial cure. The sleeve may also be formed by the contact of the foaming resin with the mandrel to form a non-porous membrane thereabout. In this case, the sleeve and cake are formed from the same resin but possess different structural properties in the product. While the foregoing method could be used to form a single polishing pad, the manufacturing advantages offered by molding a large volume cake with a window formed by the sleeve extending therethrough and then subdividing the cake are not experienced.
The polishing pad formed in accordance with the subject invention is characterized by the presence of a non-porous resin sleeve interposed between the body of the pad and the optical window. The sleeve is cross-linked to both adjacent resin elements thereby providing structural integrity, enhanced durability and protection against leakage during use.
Further features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment thereof when viewed in conjunction with the accompany drawings.